The invention relates generally to flow sensor assemblies for determining the flow rate of a flow through a conduit and, more particularly, to correlated flow sensor assemblies for determining flow rates.
Sleep apnea is a condition that affects millions of people and, if undiagnosed and untreated, may cause serious health complications. Accordingly, accurate and reliable sleep apnea monitors are needed. A number of different sensing techniques are employed to monitor air path flow. Known sleep apnea monitoring systems comprise a variety of flow and/or pressure sensors. These sensors may include anemometers, as well as ultrasonic, differential-pressure or thermistor flow sensors. The flow and/or pressure sensors can be placed in the flow conduit, in the patient/monitoring system interface (for example, in a mask fitted on the patient's nose) or in the control electronics box.
Monitoring air path flow for sleep apnea applications presents a number of challenges, including providing sufficient sensitivity over a large dynamic range, as the air flow may vary in a range of about 0-200 liters per minute. However, existing flow sensor assemblies may be sensitive to noise parameters, such as temperature, moisture and density fluctuations. In addition, air flows in both directions through the flow conduit, which further complicates the measurements.
It would therefore be desirable to provide flow sensor assemblies that can operate under large moisture fluctuations and that exhibit the desired sensitivity (typically a fraction of a liter per minute) over a large dynamic range of air flow rates (for example, over a range of about 0-200 liters per minute). It would further be desirable for gain stability and sensor noise to have no effect on the measured flow rates.